1. Field of the Invention
The invention relates to a method of testing a magnetoresistive sensor; and, more specifically, the invention relates to a method for testing a magnetoresistive sensor for susceptibility to a polarity reversal.
2. Description of the Background Art
Most of the data in contemporary computer systems are stored on disk drives using magnetic recording of digital information. A disk drive has at least one rotatable disk with discrete concentric tracks of data. Each disk drive also has at least one recording head typically having a separate write element and read element for writing and reading the data on the tracks.
A magnetoresistive sensor is used as the read element in most contemporary disk drives. A magnetoresistive sensor includes a sandwich of layers, also known as a sensor stack, including a ferromagnetic pinned layer, a nonmagnetic electrically conducting layer, and a ferromagnetic free layer. The resistance of the magnetoresistive sensor changes with respect to the direction and magnitude of an applied magnetic field such as the field from a written magnetic transition on a disk. To detect the change in resistance, sense current is passed through the sensor through electrical leads. Typically, hard bias material is disposed in layers near the ends of a sensor stack forming permanent magnets which impose a stabilizing magnetic biasing field on the sensor stack.
The sensor stack in some magnetoresistive sensors includes a relatively thick layer of antiferromagnetic material (AFM) such as an alloy of platinum manganese disposed adjacent to the pinned layer. The AFM layer helps to maintain the direction of magnetization in the pinned layer. Alternately, in a self-biased sensor, the AFM layer may be omitted. Instead, the pinned layer is formed such that the direction of magnetization in the pinned layer is held in place principally with stress induced magnetoanisotropy. These self-biased magnetoresistive sensors have the advantages of a smaller read gap and less sense current shunting through the AFM layer.
All sensors, and particularly self-biased sensors, are subject to a reversal of the direction of magnetization in the pinned layer. A magnetization reversal occurs when the direction of magnetization in the pinned layer is rotated approximately 180 degrees. A sensor which has experienced magnetization reversal in the pinned layer will exhibit a polarity reversal in the readback signal. Thus, the readback signal from a written transition which was originally positive will become negative if a polarity reversal in the pinned layer has occurred. Typically, the recorded information of the servo system is polarity sensitive. In some applications, the synchronization field recorded on the data track is also polarity sensitive. Accordingly, a disk drive having a magnetoresistive sensor which has undergone a magnetization reversal in the pinned layer will no longer function properly. A disk drive user may no longer be able to access the data stored on the disk drive.
Most conventional and self-biased magnetoresistive sensors have a low probability of a magnetization reversal. However some sensors are more susceptible, and the consequences of a magnetization reversal is severe. Accordingly, a method to detect if a sensor is susceptible to a magnetization reversal is greatly needed.